1. Field of the Invention
The present invention relates to a method of manufacturing a ceramic laminate which is adapted to provide a laminated ceramic capacitor, a laminated LC composite part, a ceramic multilayer substrate or the like, and more specifically, it relates to a method of stacking ceramic green sheets.
2. Description of the Background Art
A component such as a laminated ceramic capacitor or a ceramic multilayer substrate, for example, having laminated structure of ceramic sheets provided with an internal electrode which is interposed in at least one interface, has been generally obtained by applying a ceramic slurry on one surface of a carrier film through a doctor blade method or the like and drying the same, separating a resulting ceramic green sheet from the carrier film, printing metal paste on the ceramic green sheet by screen printing or the like and drying the same, stacking a desired number of such ceramic green sheets to obtain a laminate, and pressurizing the laminate along the direction of stacking under pressure of hundreds of kilograms to several tons per cm.sup.2 and a temperature of 40.degree. to 80.degree. C. This laminate is cut if necessary, and thereafter fired to obtain a sintered ceramic laminate.
FIG. 11 is a sectional view showing a ceramic multilayer substrate 1 which is obtained basically through the aforementioned steps. Referring to FIG. 11, the substrate 1 comprises a plurality of ceramic layers 2 to 9 and internal electrodes 10 to 14 which are provided in interfaces between specific ones of the ceramic layers 2 to 9. A plurality of conductor films 16 to 18 are formed on one major surface 15 of the substrate 1. The conductor film 16 is electrically connected with the internal electrode 10 through an internal hole 19 passing through the ceramic layer 2. Further, two through hole connecting portions 20 and 21, for example, pass through the substrate 1, so that the through hole connecting portion 20 electrically connects the conductor film 17 and the internal electrodes 10, 12 and 14 with each other while the other through hole connecting portion 21 electrically connects the conductor film 18 and the internal electrodes 11 and 13 with each other.
When the ceramic multilayer substrate 1 shown in FIG. 11 is obtained by the aforementioned method, however, the following problems are encountered.
A laminate of ceramic green sheets prepared to obtain the laminated structure of the ceramic layers 2 to 9 is pressurized in a stage before firing, as hereinabove described. Extremely high pressure is applied for such pressurization as understood from the aforementioned numerical values, to cause distortion of the green ceramic laminate, including metal paste films for providing the internal electrodes 10 to 14. In general, such distortion is so nonuniform that it is difficult to obtain the green ceramic laminate, including the metal paste films, in designed dimensions. Thus, such a green ceramic laminate frequently deviates from the designed dimensions, causing poor yields. Nonuniform distortion of the green ceramic laminate in the aforementioned pressurizing step causes a significant problem particularly in the ceramic multilayer substrate 1 shown in FIG. 11, for example, which requires high positioned accuracy for the internal electrodes 10 to 14, the internal hole 19 and the like.
Further, dried ceramic green sheets for providing the ceramic layers 2 to 9 are basically different in material composition from dried metal paste films for providing the internal electrodes 10 to 14. Sufficient junction strength cannot be attained by compressing such members of basically different materials under mechanical pressure, and hence the finished product obtained upon firing exhibits a reduced rupture strength and resistance against thermal shock. In extreme cases, delamination may result.
In order to obtain large capacitance in a laminated ceramic capacitor, for example, the ceramic layer located between each pair of internal electrodes is most typically reduced in thickness. Referring to FIG. 12, a ceramic green sheet 22 is so thinned that its physical thickness 23 is substantially equal to physical thickness 25 of a metal paste film 24 upon drying. When such ceramic green sheets 22 are stacked with each other, the thickness 25 of the metal paste film 24 partially formed on one major surface of each ceramic green sheet 22 cannot be neglected such that, as shown in FIG. 13, relatively large stress remains in portions 27 and 28 corresponding to edges of the metal paste films 24 upon pressurization of a laminate 26 of the ceramic green sheets 22. Such stress causes delamination or insufficient resistance against thermal shock after firing.
FIGS. 14 and 15 show the so-called print lamination method, which is adapted to solve the aforementioned problem. This method basically repeats the steps shown in FIGS. 14 and 15. Referring to FIG. 14, for example, a squeegee 31 is driven along the direction of the arrow to act on a metal paste member 30 which is placed on a screen 29, thereby to form a metal paste film 33 for providing an internal electrode. Then, as shown in FIG. 15, another squeegee 36 is driven along the direction of the arrow to act on a ceramic slurry member 35 which is placed on a screen 34, thereby to form a green ceramic layer 37 for covering the metal paste film 33. Respective ones of such ceramic slurry members 35 and metal paste members 30 are repeatedly printed and dried to obtain a desired laminate.
However, the aforementioned print lamination method exhibits the following problems.
First, the green ceramic layer formed by printing exhibits a higher degree of defects than a sheet formed through casting by the doctor blade method or the like. It is necessary to reduce the degree of defects by repeating the printing step a plurality of times particularly for forming a green ceramic layer to be held between internal electrodes. This results in reduced productivity.
Further, the thickness of the ceramic layer located between the internal electrodes must be adjusted by merely controlling printing conditions. Such control is relatively difficult in practice. In addition, the thickness of the ceramic layer cannot be easily managed if printing must be repeated a plurality of times in order to obtain one ceramic layer as hereinabove described. Thus, the capacitance deviates from a designed value and reduces yields.
In order to attain a sufficient degree of mechanical strength (as is required for a laminated ceramic capacitor, for example), upper and lower portions of the laminate must be covered with ceramic outer layers having no internal electrodes. However, the thickness of the ceramic layer which can be formed by screen printing is no more than several to tens of micrometers, if these layers must be hundreds of micrometers thick, the number of printing operations required is extremely high thereby reducing productivity.
In both of the aforementioned method of forming ceramic green sheets through casting by the doctor blade method or the like and the method of forming green ceramic layers by the print lamination method, pores and pinholes may be defined in the ceramic layers upon reduction in thickness. Thus, a capacitor may be reduced in voltage resistance, for example, or, in an extreme case, a short may be created across the internal electrodes.